This invention relates to an improved electrophotographic reproduction process using a photoconductive insulating layer on a conductive support to make a lithographic printing plate. Particularly, it relates to a process for removal of unwanted fused toner as a last step prior to printing and to the product thereby produced.
In the practice of lithography using bimetallic plates, such as copper laminated to aluminum, it is necessary to use a photoresist layer to prepare the image. The photoresist is coated over the surface of the copper, imaged under a mask and developed. After development, the residual photoresist is present in imagewise configuration leaving a pattern of exposed copper corresponding to the nonimage. At this point, the plate is etched with, for example, ferric chloride solution to dissolve the unprotected copper. After rinsing, the residual photoresist is stripped with a suitable solvent, leaving an oleophilic copper image on a hydrophilic aluminum support. The copper may also be treated with dilute acids to enhance its oleophilicity.
Lithographic offset plates have been prepared by electrophotographic methods. Such plates are normally composed of a photoconductive material such as zinc oxide, cadmium sulfide or certain organic compounds dispersed in an ink-repelling binder and coated on a suitable base material such as paper, metal or a film. These plates are imaged by the normal electrophotographic process involving forming an electrostatic charge on the surface of the plate, exposing the charged plate on an electrically conductive support to an image pattern of electromagnetic radiation to leak away the charge on the areas struck by light, developing the resulting electrostatic image pattern by contact with an electroscopic liquid or solid developer, and fixing the developed image by drying or heating. The resultant imaged plate may be then used as a master for offset lithographic printing.
Following the aforementioned fixing step, the fused or fixed toner lies in imagewise configuration upon a thin adherent, continuous layer of photoconductive material which in turn lies upon the conductive support. In those cases where the non-toned photoconductor surface is not hydrophilic, the toner covered image may be treated with a decoating solution, which removes photoconductive coating without removing that part of the coating masked by fixed toner to reveal the nonimage areas. At this point, the plate is put on the press whereupon the oleophilic toner over the image areas attracts ink and transmit ink to the blanket in offset printing (or directly in direct lithography) while the decoated support areas attract water and repel ink as in normal lithography.
Plates may be charged, exposed, developed and decoated in automatic machinery using incoherent light as in U.S. Pat. No. 3,999,511 or in fully automatic laser exposing machinery as described in U.S. Pat. No. 4,149,798.
It has been found that sporadically, and without explanation, the printed image may become mottled in large black areas at the outset of a printing run until about three thousand impressions have been made. This phenomenon may be present with either dry toned or liquid toned plates, but persists somewhat longer, when it occurs, with liquid toned plates. In any case, the mottled prints are unsightly and must be discarded, causing an economic loss.
On some occasions yellow or other light colored inks may slowly dissolve toner from the image on the plate during printing with consequent degradation of purity of color on the printed sheet.
In critical printing applications, dot gain is to be avoided. It is believed in some quarters that the toned image contributes undesirably to dot gain in these cases.
Accordingly, it is an object of this invention to provide a product and a process which eliminates the problem of mottled prints on the press.
It is another object to provide purer colors in color printing. Still another object is to minimize dot gain in critical printing.